Target contains concave polygons
From the manual, at https://docs.blender.org/manual/en/dev/modeling/modifiers/deform/surface_deform.html , the target

Must not contain faces with collinear edges.

You have at least two faces with three collinear points (what I've heard called a degenerate quad).  That's what you can't have.  I've selected one in the image below.

To add to this, the quickest way to fix this problem (and non-destructively to boot!) is to use a triangulate modifier.  On your model, if you put a triangulate modifier on your plane, after the cloth modifier, your fish will bind properly.
That's not always going to be the case; surface deform gives that warning inappropriately sometimes, when the problem is something other than concave polys.  (It's interesting to me that binding a Suzanne to a Suzanne is basically impossible...)  But in general, whenever you want to use a surface deform and can't bind, the first thing to do is triangulate the target.